An integral part of microelectronic fabrication is the use of photoresists to transfer an image from a mask or reticle to the desired circuit layer. After the desired image transfer has been achieved, the photoresist is removed by stripping before proceeding to some subsequent process step. Since about 1980, amides and mixtures of amides with various co-solvents have routinely been used for this stripping step, see for example U.S. Pat. Nos. 4,395,479; 4,428,871 and 4,401,748, issued to Ward et al.
Depending upon the type of pre-stripping processing that is performed while the photoresist is in place, the photoresist polymer may be cross-linked or hardened to the extent that amide-based solvents will no longer effectively strip the photoresist. Since about 1985, the use of amide mixtures containing additional alkaline components, such as organic amines (U.S. Pat. Nos. 4,592,787 Johnson; 4,765,844 Merrem; 4,617,251 Sizensky; WO 87/05314 Turner; U.S. Pat. No. 4,791,043 Thomas et al.) or quaternary ammonium hydroxides (U.S. Pat. No. 4,776,892 Steppan et al; 4,744,834 Haq; WO 88/05813 Martin) were introduced to facilitate the removal of such hardened photoresists.
The use of these alkaline strippers on microcircuit substrates containing metal films, particularly aluminum or various combinations or alloys of active metals such as aluminum or titanium with more electropositive metals such as copper or tungsten, has proven problematic. Various types of metal corrosion, such as corrosion whiskers, pitting, notching of metal lines, have been observed due, at least in part, to reaction of the metals with alkaline strippers. Further it has been shown, by Lee et al., Proc. Interface '89, pp. 137-148, that very little corrosive action takes place until the water rinsing step that is required to remove the organic stripper from the wafer. The corrosion is evidently a result of contacting the metals with the strongly alkaline aqueous solution that is present during rinsing. Aluminum metal is known to corrode rapidly under such conditions--Ambat et al., Corrosion Science, Vol. 33 (5), p. 684, 1992.
In the past it has been proposed to circumvent this problem by employing intermediate rinses with non-alkaline organic solvents such as isopropyl alcohol. It would be desirable to avoid the expense and possible safety, chemical hygiene, and environmental consequences of such an intermediate rinse by eliminating this additional rinse by providing an effective stripper that does not require it.
In U.S. Pat. No. 5,102,777, issued on Apr. 27, 1992 to Lin et al., it is proposed to combat such corrosion by employing a positive photoresist stripper composition which includes a solvent system having solubility parameters which fall within a range of from about 8.5 to about 15 in an amount which falls within a range of about 65% to about 98%. In addition, an amine is present in an amount of from about 2% to about 25% and also a fatty acid having 8 to 20, preferably 10 to 16, carbon atoms in an amount of from about 0.1% to about 10% such that the amount of amine and fatty acid is selected to provide a pH of from about 6 to 9.5, preferably 7 to 8.5. However, while the positive photoresist stripping compositions disclosed in said patent were found to provide some beneficial effect vis-a-vis metal corrosion, these disclosed stripping compositions were not able to be used to satisfactorily strip hard-to-strip cross-linked or hardened photoresist resins. Moreover, in the one stripping formulation of the patent which is capable of stripping cross-linked photoresist resins, one is not able to prevent or avoid metal corrosion. These deficiencies and drawbacks of the stripping compositions of the Lin et al. patent are demonstrated in the Comparative Example set forth hereinafter in this specification.
It is therefore an object of this invention to provide alkaline-containing photoresist strippers which do not require intermediate rinses to avoid metal corrosion and yet the photoresist strippers are still highly effective in stripping cross-linked or hardened photoresists. A further object of this invention is to provide such improved non metal corroding, alkaline-containing photoresist compositions without any undue adverse effect on strip rate of the photoresist for cross-linked or hardened photoresists.